1. Field of the Invention
The present invention relates to a pipe forming apparatus which uses bending rolls, as well as to a pipe forming method and pipe formed by the apparatus or method. More particularly, the present invention relates to a pipe forming method which uses bending rolls including a pair of lower rolls arranged at one side of a sheet material and an upper roll arranged at the other side of the sheet material intermediate the pair of lower rolls, as well as to a pipe forming apparatus including the bending rolls and a pipe formed by the pipe forming apparatus or method. The pipe forming apparatus and method of the present invention are suitable for production of high-strength, thick-walled pipes.
2. Description of the Related Art
Mass production of pipes is generally carried out by using an electric welding mill when the pipe diameter is comparatively small, whereas UOE mills are used for pipes having comparatively large pipe diameters.
For small-lot production, however, a method as shown in FIG. 1 is employed for pipes having large wall thicknesses. More specifically, referring to FIG. 1, a blank sheet 10 is pressed between a bending die 22 and a pressing die 24 of a roll bender 20, and the pressing operation is repeated many times, e.g., 50 times or more, so as to bend the blank sheet into a pipe. In contrast, a method as illustrated in FIG. 2 is used when the wall thickness of the pipe is small. More specifically, referring to FIG. 2, a pyramidal roll bender 30 employs three rolls: a pair of lower rolls 32 disposed under a blank sheet 10 and driven by a motor (not shown) and an upper roll 34 disposed on the upper side of the blank sheet 10 at a position intermediate the pair of lower rolls 32. The tightening amount S, i.e., the extent to which the upper roll 34 is lowered, is adjustable. In operation, the blank sheet 10 is threaded between the upper roll 34 and the lower rolls 32 so as to be continuously bent at a curvature radius xcfx81.
A process of producing a pipe by the illustrated bending rolls is shown in FIG. 3. As the first step, cutting and groove work are conducted by means of a flame planer 40 which cuts a blank sheet by using an oxygen or other gas plasma. Then, an end bending operation is performed by, for example, a hydraulic press for bending the end of the blank sheet that cannot be bent by bending rolls. Then, bending is effected by using, for example, a pyramidal roll bender 30 having three rolls as shown in FIG. 2. Subsequently, inner/outer surface welder 46 performs tack welding and welding of inner and outer surfaces. Then, end face milling is performed by an end face mill (not shown). The pipe thus formed is then shot-blasted in a shot-blast apparatus (not shown) and sent for testing and inspection. In some cases, bend correction is performed subsequent to roll bending or tack welding, in order to enhance the circularity of the pipe. Such bend correction is conducted substantially in the same way as the roll bending.
The production of pipes using bending rolls is suitable for multi-variety and small-quantity production, because it is adaptable to a variety of pipe diameter through adjustment of the amount S of tightening of the upper roll 34. In addition, this type of pipe forming method is superior to methods using UOE mills, because of lower installation cost, and so forth.
On the other hand, this pipe forming method tends to allow an end gap 12A to occur in the center of the resulting pipe 12 as shown in FIG. 4, due to the difficulty of preventing deflection of the bending roll, especially the upper roll 34. Therefore, this method is not suitable for use in the production of high-strength, thick-walled elongated pipe, which places a heavy load on the rolls tending to cause deflection of the rolls.
This shortcoming arises because the upper roll 34 is supported only at its opposite ends and the outside diameter Dwu of the upper roll 34 is limited by the diameter of the product pipe, such that the roll diameter must be smaller than the pipe diameter due to a geometrical requirement. On the other hand, deflection of the lower rolls is controllable because backup rolls may be placed in support of the lower rolls at suitable positions.
The pipe production method using a press bender is more suitable for the production of thick-walled pipes than the pipe-production method using a roll bender, but has a drawback in that the production efficiency is low due to the need for many pressing cycles, resulting in an elevated cost of production.
FIG. 5 shows a pipe forming method which overcomes the above-described shortcoming of roll benders and which is disclosed in Japanese Kokai No. 53-128562. In this method, pressure is applied during forming a pipe to the upper roll 34 from the upper side thereof by means of a backup beam 36 via backup rolls 35.
With this method, deflection of the upper roll 34 is reduced so that the formation of the end gap is suppressed, thereby to facilitate production of high-strength, thick-walled elongated pipes.
As an alternative measure, it is possible to use an upper roll of a greater diameter in accordance with an increase in the diameter of the pipe to be produced, thereby enabling production of high-strength, thick-walled elongated pipes, by decreasing deflection of the upper roll.
These known methods, however, are not suitable for use in the production of relatively small-diameter pipes, although they are effectively used in the production of large-diameter pipes. This is because there is insufficient space available between the upper roll 34 and the inner surface of the pipe 12 for accommodating a backup beam 36 when the pipe diameter is relatively small, and a small-diameter upper roll thus must be used alone, since, as stated before, the outside diameter Dwu of the upper roll 34 is limited by the inside diameter of the pipe to be produced. For these reasons, production of small-diameter pipes tends to be accompanied by creation of an end gap, due to lack of a technique for suppressing deflection of the upper roll, such that the upper roll is often bent beyond an allowable limit.
Accordingly, an object of the present invention is to described before, by improving the product pipe configuration through reducing end gap which tends to appear at a longitudinally central portion of the pipe, without causing the roll to be bent or deflected beyond an allowable limit.
To this end, according to one aspect of the present invention, there is provided a pipe forming apparatus of the type using bending rolls including a plurality of rolls arranged on one side of a sheet material, and a counter roll arranged at the other side of the sheet material, comprising: a roll spacing setting device that sets the roll spacing of the pair of rolls to a range greater than the sum of the diameter of the counter roll and the diameter of one of the pair of rolls; and a device that sets the tightening amount of the counter roll relative to the pair of rolls to a range greater than the radius of one of the pair of rolls.
The above object of the invention is achieved by forming the sheet material into a pipe with the pipe forming apparatus while setting the roll spacing between the pair of rolls to a value greater than the sum of the diameter of the counter roll and the diameter of each of the pair of rolls, and setting the amount of tightening to a value greater than the radius of each of the pair of rolls.
In order to achieve the above object, the present invention in its second aspect provides a pipe forming method for forming a pipe using bending rolls including a plurality of rolls arranged on one side of a sheet material, and a counter roll arranged at the other side of the sheet material, the pipe forming method comprising: effecting pipe forming work on the sheet material such that the spacing L of the pair of rolls satisfies the following expression:
(Dp+Dwl) greater than Lxe2x89xa70.85(Dp+Dwl)xe2x80x83xe2x80x83(1)
where Dp represents the outside diameter of the product pipe and Dwl represents the diameter of one of the pair of rolls.
To achieve the foregoing object, the present invention in its third aspect provides a pipe forming method for forming a pipe using bending rolls including a plurality of rolls arranged on one side of a sheet material, and a counter roll arranged at the other side of the sheet material, the pipe forming method comprising: preparing the sheet material having leading and trailing bent end regions bent beforehand over a length not smaller than ⅕ of the entire circumference of the pipe to be produced; and effecting roll bending such that the length of the regions bent by the bending rolls is less than ⅗ the entire circumference of the pipe to be produced.
These and other objects, features and advantages of the present invention will become clear from the following description of preferred embodiments when read in conjunction with the accompanying drawings.